1. Field of the Invention
The present invention relates generally to DC-DC power conversion.
2. Background of the Invention
DC-DC power converters are power-processing circuits that convert an unregulated DC input voltage to a regulated DC output voltage, usually at a different level, for powering a load. A vast variety of topologies for DC-DC converters have been introduced over the years, but not all are suitable for delivering the low voltage and high current outputs that are now required by microprocessor, memory and other integrated circuit loads. Further, the need for small size and high efficiency places additional limitations on the available topologies. Small size equates to high power density, and power density is the ratio of output power capability to converter volume.
To achieve high power density, the power loss must be low, or the operating temperature will increase, and additional thermal management devices, such as heatsink dissipators, may be required. The use of such devices defeats the objective to obtain high power density. To avoid heatsinks under normal operating conditions, the conduction losses must be minimized, and synchronous rectifiers have been shown to greatly improve rectification efficiency.
Synchronous rectifiers require a control signal to drive the device to a low resistance state and provide very low loss conduction, but they also include an internal diode, which can conduct current, albeit with higher losses. A proper control strategy is needed to ensure that the internal diode does not conduct. Synchronous rectifiers can also conduct in reverse, and this could produce a short circuit, so the controlling circuit must be carefully designed.
A fundamental DC-DC power conversion topology is the single-ended forward converter shown in FIG. 1a. This topology, when controlled by a constant frequency, pulse-width-modulation (PWM) control circuit 10, provides excellent regulation and fast response time. In operation, the primary switch 11 is turned ON to apply the source voltage Vin to the transformer 12. Immediately, a secondary voltage appears, and current flows simultaneously in the primary winding 13 and secondary winding 14, and energy is transferred forward. The secondary load current flows through diode 15, and diode 16 is reverse biased at this time. The difference between the secondary winding voltage and the output voltage Vout appears across the filter inductor 17, and energy is stored in the inductor 17 during this ON period. In addition, the inductor 17 limits the rate of change of current during the ON period.
When switch 11 is turned OFF, the current in the secondary winding 14 vanishes, but load current continues to flow through diode 16 and inductor 17, and the stored energy in the inductor 17 provides continuity of current to the filter capacitor 18 and output Vout. The current in the transformer primary winding 13 also vanishes except for a small amount of magnetizing current. Various methods have been disclosed to reset the transformer core during the OFF period, and these are well known to those skilled in the art. The primary and secondary winding voltages will reverse during reset, and diode 15 is reverse biased disconnecting the load (not shown) from the transformer 12.
The single-ended circuit of FIG. 1a is not optimal, and one deficiency is that energy for the entire switching cycle must be drawn from the source (Vin) during the ON period of the primary switch 11, and an equivalent period of time is required for the OFF period of the primary switch to allow the core to reset. The single pulse of high current followed by a long dead time results in a high RMS current and excessive conduction loss in the primary circuit, thus limiting the topology to low power applications. Furthermore, the output voltage Vout is the average value of the pulsed waveform that appears on the secondary winding 14, and due to the extended dead time, each rectifier (i.e., diodes 15, 16) experiences a peak reverse voltage much higher than the average. Because rectification is only accomplished approximately half of the time, i.e. during the ON period of the primary switch, the topology is known as half-wave.
These deficiencies are almost entirely removed by the double-ended topology of FIG. 1b. The double-ended topology operates much like two overlapping single ended circuits and has similar control and response characteristics. The power converter of FIG. 1b includes a second primary switch 21, which is controlled ON during the time switch 20 is OFF. In operation, switch 20 first connects the primary winding 22 to input capacitor 23, and then switch 21 connects the same winding 22 to input capacitor 24. This results in an alternating voltage across the primary winding 22. The voltage across each of the input capacitors 23, 24 will be one-half the source voltage Vin.
Energy is transferred to the secondary windings 26, 27 during the ON period of each primary switch 20, 21, and the core flux, which increases during the first ON period, is reset during the subsequent ON period. A dead time for reset is not required. However, dead time may be used along with a PWM regulation technique, provided by PWM control circuit 25. This control time can be varied from zero to a full half-cycle. With a double-ended topology, two current pulses of lower magnitude are drawn from the source during each switching cycle, and the primary winding 22 carries bipolar current with an improved RMS value.
The half-bridge topology of FIG. 1b is shown with two secondary windings 26, 27, and each is connected to one of the rectifying diodes 28, 29. The diodes 28, 29 alternately conduct current from their respective secondary windings 26, 27 when they are forward biased, and the rectification is known as full-wave. The more continuous current to the output Vout reduces the requirement for energy storage during any dead time which may occur, and the inductor 30 consequently may be made smaller.
One known variation to these topologies is to translate the filter inductor to the primary circuit. A single ended circuit according to this variation is shown in FIG. 2, and the core reset mechanism is not shown. The primary winding 32 and inductor winding 33 now carry primary current, which is typically less than load current. In addition, as before, the inductor 33 stores energy during the ON period of the primary switch 31. However, to permit discharge of this energy during the OFF period, a second winding 35 must be added to the inductor and connected through a diode 37 to the output Vout. Effectively, the inductor has become a second flyback transformer with its primary 33 connected in series with the primary winding 32 of the first transformer 39. The ratio of the primary 33 to secondary 35 turns on the inductor may be identical to the ratio of the primary 32 to secondary 34 turns on the transformer 39.
With the filter inductor 33 located in the primary circuit the input voltage drops across it, and a reduced voltage is applied to the transformer 39. In operation, the secondary windings 34, 35, and diodes, 36, 37 are connected directly to the output voltage Vout, and the winding and reverse diode voltages are limited to the magnitude of the output voltage Vout. A constant frequency PWM control technique can be applied to the primary switch 31 to regulate the output voltage Vout. Double-ended topologies pursuant to this variation, including a half-bridge type that is analogous to FIG. 1b, are also known.
Still, none of the above topologies define a suitable control method when synchronous rectifiers are used to reduce rectification losses. Accordingly, there exists a need in the art for a power conversion topology and control technique that is compatible with synchronous rectification and yet capable of satisfying the requirements for high power density and low voltage, high current output.
In one general respect, the present invention is directed to a DC-DC power converter. According to one embodiment, the converter includes first and second transformers, and a double-ended input circuit, including first and second primary switches, for generating an alternating voltage across the primary windings of the first and second transformers. The converter also includes a control circuit (such as, for example, a PWM control circuit) for controlling the primary switches such that the primary switches are simultaneously OFF for a first time period during a switching cycle of the converter. In addition, the converter includes first and second synchronous rectifiers. The first synchronous rectifier is coupled to the secondary winding of the first transformer and the second synchronous rectifier is coupled to the secondary winding of the second transformer. The first synchronous rectifier may be for rectifying a voltage across the secondary winding of the first transformer and the second synchronous rectifier may be for rectifying a voltage across the secondary winding of the second transformer. Further, the control circuit is for controlling the first and second synchronous rectifiers such that the first and second synchronous rectifiers are simultaneously ON for a second time period during the switching cycle. The first time period and the second time period may be the same, thereby providing simultaneous conduction of the synchronous rectifiers when the primary switches are simultaneously OFF.
According to various embodiments of the power converter, the control circuit may control the first synchronous rectifier such that when the first primary switch is ON, the first synchronous rectifier is simultaneously ON such that the first transformer transfers energy forward. Similarly, the PWM control circuit may control the second synchronous rectifier such that when the second primary switch is ON, the second synchronous rectifier is simultaneously ON such that second transformer transfers energy forward.
Additionally, the control circuit may control the first synchronous rectifier such that the first synchronous rectifier may be turned ON prior to the time the first primary switch is turned on, such that there may exist a delay therebetween. Similarly, the control circuit may control the second synchronous rectifier such that the second synchronous rectifier may be turned ON prior to the time the second primary switch is turned on, with delay therebetween as well. In addition, the synchronous rectifiers may conduct to discharge energy stored in their associated transformers pursuant to a flyback transformer mode of operation.
Implementations of the power converter, according to other various embodiments may include one of more of the following features. For example, the control circuit may include a PWM control circuit. In addition, the double-ended input circuit may include, for example, a half-bridge input circuit, a full-bridge input circuit or a push-pull input circuit. The primary windings of the first and second transformers may be connected in series, as may the secondary windings. In addition, the turns ratios of the first and second transformers may be the same. According to another embodiment, the synchronous rectifiers may be self-driven.
Benefits that may be realized with a power converter according to the present invention include high density with low voltage/high current output. These and other advantages of the present invention will be apparent from the description to follow.